A group that opposes Nebraska's death penalty says the state won't be able to import lethal injection drugs without violating federal law and exposing itself to additional legal challenges.

The Nebraskans for Public Safety coalition took issue Wednesday with assertions that state officials are working with the federal government to try to obtain the necessary drugs.

The group released written comments from University of Nebraska law professor Eric Berger, noting that federal law makes it "very clear" that the drug sodium thiopental cannot enter the country legally for executions.

Berger says states will likely face more difficulty carrying out executions in the future.

Nebraska lawmakers abolished capital punishment in May, prompting a petition drive that suspended the repeal until voters decide the issue in November 2016.